Science Puzzle
The Mpemba Effect
Erasto Mpemba, a Tanzanian student, noticed in 1963 that hot ice cream mix froze faster than cold mix in the school freezer. His physics teacher told him he must be mistaken. He asked physicist Denis Osborne to investigate.
Under some conditions, hot water really does freeze faster than cold water placed in the same freezer at the same time. How can this be?
The Answer
This is the Mpemba Effect and it is real, though the full explanation is still actively debated by physicists. Several mechanisms have been proposed and may all contribute depending on the conditions.
Hot water evaporates more, reducing its mass so there is less to freeze. Dissolved gases escape from hot water, and dissolved gases can affect the freezing point and thermal conductivity. Hot water develops convection currents that transfer heat more efficiently to the container walls. The thermal gradient between hot water and the freezer walls is steeper, driving faster heat loss.
A 2016 study proposed that the hydrogen bonds in hot water store more energy and release it more readily during cooling. A 2020 study suggested the effect may depend critically on experimental conditions and is not always reproducible.
The value of the Mpemba Effect as a teaching tool is not its definitive answer but its lesson: a teenager's careful observation, dismissed by an expert, turned out to be pointing at a real and still unresolved problem in physics.
The principle: The Mpemba Effect. Under some conditions hot water freezes faster than cold water. Several mechanisms contribute (evaporation, dissolved gases, convection, supercooling) and the full explanation remains an open question.